Variable selectivity device



Sept. 16, 1941.

w. CARLSON VARIABLE SELECTIVI'I Y DEVICE Filed Nov. 5, 1934 2Sheets-Sheet 1 Manual Se/ecf/i/ifv (0/777'0/ Tuner --"""'-1 INVENTOR.

WENDELL L. CARLSON ATTORNEY.

Sept. 16, 1941.

w. L. CARLSON 2,256,073

VARIABLE SELECTIVITY DEVICE Filed Nov. 5, 1934 2 Sheets-Sheet 2 ReceiverOurjouf Vo/fage CharaCfer/Lsf/t D C,

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IHHI INVENTOR.

WENDELL L. CARLSON ATTORNEY.

Patented Sept. 16, 1941 T OFFICE VARIABLE SELECTIVITY DEVICE \Vendell L.Carlson, Haddonfield, N. J., assignor to Radio Corporation of America, acorporation of Delaware Application November 3, 1934, Serial No. 751,305

8 Claims.

My present invention relates to selectivity adjustment devices for radiobroadcast receivers, and more particularly to an improved device forreadily adjusting a broadcast receiver to operate either with a highdegree of fidelity or with a high degree of selectivity.

One of the main objects of the present invention is to provide animproved variable selectivity device which is capable of adjusting thedegree of selectivity of a radio broadcast receiver, and whichselectivity device additionally includes means for regulating the audiofrequency transmission band'of the receiver whereby the receiver outputaudio frequency characteristic is changed when the degree of selectivityof the receiver is adjusted.

Another important object of the invention is to provide in a radiobroadcast receiver which has a substantially flat output voltagecharacteristic over a wide audio frequency range, a variable selectivitydevice which is capable of adjusting the receiver so as to operate witha high degree of fidelity or with a high degree of selectivity, theselectivity device having operatively associated with it an audiofrequency transmission control element which is adapted to attenuate thetransmission of the higher audio frequencies when the selectivity deviceis adjusted to increase the degree of selectivity of the receiver.

Another object of the invention is to provide an adjustable selectivitycontrol for a radio receiver, which control is independent of the tuningof the receiver, the selectivity control being adapted to operate atdifferent rates in succes- I sive tuned networks of the receiver, andaddition- 1 ally including a tone control element for varying theattenuation of the higher audio frequencies during selectivityadjustment.

Still other objects of the invention are to improve generally theefficiency of variable selectivity devices for high fidelity radiobroadcast receivers, and more particularly to provide selectivitycontrol arrangements for receivers which are not only reliable inoperation, but economically constructed and assembled in the radioreceiver.

The novel features which I believe to be characteristic of my inventionare set forth in particularity in the appended claims, the inventionitself, however, as to both its organization and method of operationwill best be understood by reference to the following description takenin connection with the drawings in which I have indicateddiagrammatically several circuit organizations whereby my invention maybe carried into effect.

In the drawings:

Fig. 1 diagrammatically shows a superheterodyne receiver embodying theinvention,

Fig. 2 graphically shows the characteristics of the radio frequencynetworks when radio frequency selectivity is varied,

Fig. 3 shows the output voltage characteristic when selectivity isvaried,

Fig. 4 shows a modified form of the invention.

Referring now to the accompanying drawings, wherein like referencecharacters in the different figures designate similar circuit elements,there is shown in Fig. 1 a superheterodyne receiver of conventionaltype, the various networks being schematically designated since thoseskilled in the art are fully aware of the constructional detail of thenetworks shown.

The receiver embodies the usual signal collector- A, which is shown as agrounded antenna circuit, the signal collector being coupled to thetunable input circuit of a radio frequency amplifier. This amplifier mayembody one, or more, stages of tunable radio frequency amplification,and the amplifier stages may employ electron discharge tubes of any typeadapted for such purposes and well known in the art. The amplified radiofrequency signal energy is then impressed upon a first detector whichhas a tunable input circuit, and there is impressed upon the firstdetector locally produced oscillations from the usual local oscillator.The tuning of the local oscillator is varied simultaneously with thetuning of the first detector and the radio frequency amplifier.- Ofcourse, those skilled in the art are fully aware of combined firstdetector-local oscillator networks wherein a single tube can be employedfor performing the functions of the separate local oscillator and firstdetector. The legend "Tuner designates the uni-control tuning elementwhich mechanically couples the rotors of the various variable tuningcondensers which have been described.

The intermediate frequency energy produced in the tuned output circuit lof the first detector is impressed upon the tuned input circuit of I thefollowing intermediate frequency amplifier tube 2. The reference letterM1 designates the coupling network between the first detector and theintermediate frequency amplifier 2. The tuned secondary circuit of thecoupling network M1 is designated by the numeral 3, and it is to beunderstood that circuits l and. 3 are not only maintained fixedly tunedto the operating into contact with any one of four contact elementsdesignated by the reference letters A-BC--D,

V The contact A is connected to the coil 5 of the input circuit 3through a path including .a resistor 6 and a coil 1. The coil 1. isclosely coupled to the coil 8 of the tuned output circuit I of the firstdetector. Coils 8 and 5.are relatively loosely .coupled. Contacts B, C'and D are connected together, and contact B is connected by lead. 9

to the low alternating potential side of coil 5.

The adjustable tap 4 is connected by lead 10 to the grounded side of thetuning. condenser of circuit 3.

The plate circuit "ofamplifier tube 2 includes a tuned output circuit IIwhich isfixedly tuned to the operating intermediate frequency. Thedegree of selectivity of. the receiving system is effectively adjustedtooperate either with a high degree of fidelity or with a high degree ofselectivity by changing the position of tap 4 from contact A to eitherof the contacts B"C-D. This general type of variable selectivity devicehas been described and claimed in my U. S. Patent No. 1,871,405 ofAugust 9, 1932. It is pointed out in the patent that when broadcast,

stations are operated atfrequencies set apart by 10 kilocycles it isdiflicult to operate receiving apparatus with high 'quality'and sharpselectivity without reverting to single side band transmission. It'is,also, shown in the patent that the best quality of reception is obtainedwith a high fidelity adjustment for local reception, and with arelatively poor fidelity and high selectivity adjustment for distancereception through interference. There was, therefore, provided thedevice shown operatively associated with coupling network M1 forimparting to the coupling network M1 at substantially band passcharacteristic when the tap '4 is connected to contact A. This arises byvirtue of the relatively close coupling between coil 8 and the auxiliarycoupling coil .1.

Fig. 2 shows the type of resonance curve characteristic secured when tap4 is connected to contact A. The resonance curve characteristic withsuch a setting is broad, and has a width the changed resonance curvecharacteristic. Connection of tap 4 to contact B results in opencircuiting of coil 1 and resistor 6, and for this reason the width ofthe resonance curve is narrowed.

Additional investigation along the lines of my aforementioned patenteddisclosure has shown that more efiicient and smoother operation ofselectivity change is secured when a second selectivity adjustmentdevice is utilized, as shown in connection with the succeeding couplingnetwork M2. This coupling network comprises the tuned output circuit l Iincluding coil 8', the coil being relatively loosely coupled to the coil5 of the tuned input circuit 3' of the second detector tube 20. r Thesecond detector tube is shown as of .therdiode type, the anode of thediode being connected by an adjustable lead 2| so chosen .as to equalizethe response at different frequencies within the receiving range. Whenthe tab 4 is connected to the contact B, or contacts C-D, the resonancecurve characteristic of network M1 is changed to that shown in Fig. 2.It will be observed that the resonance curve characteristic is nowrelatively narrow, and that the higher modulation side band frequencieshave been attenuated... The receiver, in other words,

to an intermediate point on input coil 5' so as to give less load fromthe diode.

The coil 5' is connected to the grounded cathode of diode 20 through apath which includes the tuning condenser ofcircuit 3 and the loadresistor 22, the latter being shunted by a radio frequency by-passcondenser 23.

The coil 8' is relatively tightly coupled to auxiliary coupling coil 7,and contacts A and B are connected to the low alternating voltage sideof coil 1' through the resistor 6. Contacts C and D are connected to thelow alternating voltage side of coil 5' through lead 10', and theadjustable tap 4' is connected by lead 23' to resistor 22; the tap isarranged to be connected with any of the contacts associated therewith.It will be observed that the adjustable. taps 4 and 4' are arranged upona common, mechanical adjusting element 30 so that they can be operatedconjointly, and it is to be understood that the contact elements A-BCDof each of the is in contact with'either of contact members A" or B; andwhen the tap is in contact with either of members C or D. the resonancecurve for coupling network M2 is the same as the resonance curve ofcoupling network M1 when tap 4' is on either of contact members A or B.In other words, while the tap 4 has been adjusted to contact B to varythe selectivity of the receiver, the tap 4', being in contact withmember B, imparts a relatively broad characteristic to the. receiver. Itis not until tap 4 contacts with member C that network M2 possesses thesame characteristic as that of network M1. In this way the selectivitytransition is made more gradual and smoother.

It has been found that such an overlapping selectivity control device isdesirable in high fidelity broadcast radio receivers. is made ofinsulation material, and it is to be understood that adjacent taps 4'and 4 are not conductively connected through the shaft. The contactsA-BCD are preferably arranged on a circular disc, and the contactsA'B'C--D' are arranged on another disc; the shaft 30 may pass throughthe center of both discs, and the latter may be parallel. The shaft 30is adjusted independently of the Tuner, and at any desired setting ofthe latter may be varied to regulate It will be observed that The shaft30 V the degree of selectivity of the networks M1M2.

The audio component of rectified signal voltage across resistor 22 istransmitted to the first audio amplifier. The audio coupling network M3may be of any desired type; it is here shown as a resistance-capacitycoupling. A manual volume control device 40 is disposed in the networkfeeding the audio amplifier. An automatic volume control circuit,utilizing the direct current voltage component across resistor 22 ifdesired, can also be used; in that case the variable voltage would beused to regulate the gain of the radio frequency amplifiers in a mannerwell known to the prior art. Th AVC network is not shown to preservesimplicity of disclosure, but the various other receiver control devices(Tuner; Manual volume control; AVC) are referred to in order todistinguish them from the manual selectivity device of this application.

The amplified audio energy is further amplified in a second amplifier;the output of the latter may be again amplified, or reproduced by anywell known type of reproducer. Regardless of the type of audio frequencynetwork used subsequent to the second detector, it is to be understoodthat the network is constructed to give an output voltage characteristicto the re ceiver which is substantially fiat up to about 8000 cycles.This type of characteristic is graphically represented in Fig. 3 by thefull line curve Al. The curves in Fig. 3 are purely pictorial, and areto be considered as such.

In operating the variable selectivity device of this invention it isdesirable to simultaneously adjust the transmission of the higher audiofrequencies through the audio network. The effect of such desiredadjustment is represented in Fig. 3 by the dotted line curves B1C1D1. Itwill be seen that the receiver output voltage characteristic is cut offat varying frequency points as 1 the degree of selectivity is changed.Cut-off point Bl may be, for example, 6000 cycles; point C1 may be 4000cycles, and point D1 may be at 3000 cycles. This means that there willbe a gradual change of selectivity; beginning first at the radio andaudio points of the receiver, and ending with audio adjustment betweenpoints 01 and D1.

This is accomplished by providing a third tap 50 on shaft 30, the tap 50being insulated from the other taps 4 and 4', and arranging contactsA"--B"CD to be engaged by tap 50. Contact A" is free; contact B" isconnected to the grid of the second audio amplifier through condenser5i; contact C" connects to the grid through condenser 52; contact Dconnects to the audio amplifier grid through condenser 53. The threecondensers are of such relative size and arrangement that the cut-offpoints Bi-C1D1 will result when tap 50 engages contacts B"C"D"respectively. In other words audio frequencies from 3000 to 8000 cyclesare substantially attenuated when tap 50 engages contact D"; thiscorresponds to the D and D positions of taps 4 and 4' respectively.Audio frequencies above point C1 are attenuated when tap 50 engagescontact C", and this corresponds to the CC positions of taps 44. Whentaps 3- 3 engage contacts B-B, then audio frequencies above point B1 arattenuated by tap 50 engaging contact B".

The utility and advantage of varying the transmission frequencycharacteristics of both audio and radio circuits simultaneously, ornearly simultaneously, will now be explained. The overall audiofidelity. transmission characteristic is a function of both radiofrequency and audio frequency characteristics. To eliminate high audioheterodyne beat interference from side bands of adjacent channelstations which is called monkey chatter, and to reduce interference fromstatic, hiss, etc., it is desirable to sharply attenuate the over-allcharacteristic response above a predetermined desired frequency range.This may be accomplished all in one part in either the audio or radiocircuit, or it may be accomplished simultaneously in two, or more, partsof the receiver circuit. The first method would require an additionalband pass circuit network involving considerable cost to obtain thedesired attenuation. The second method as described in this applicationaccomplishes the desired result with few extra circuit parts, andconsequently lower cost.

There is another type of interference known as cross talk from adjacentchannel stations which is distinguished from monkey chatter" by the factthat interfering side bands and carrier appear on the audio detector insufiicient magnitude to rectify and produce an intelligible audioresponse independent of the desired signal. This type of interferencecan only be eliminated by attenuating the undesired signal before itreaches the audio detector, that is, by restricting the radio frequencytransmission band.

Another factor to be considered is the unavoidable high frequencydistortion which may be created at the input end of the audio circuits,for instance, at the detector. These distortion frequencies could beattenuated by restricting the audio frequency range of transmission toapproximately the same frequency range that the radio frequency circuitsefiiciently pass when the receiver over-all band width is reduced toavoid cross-talk, monkey chatter, or to produce the tone quality desiredby the listener.

For different reasons as stated above, the radio frequency and audiofrequency transmission bands should both be restricted whenever it isdesired to restrict the over-all transmission frequency band of thereceiver. This amounts to saying that the control. of the over-alltransmission frequency band should operate simultaneously on the radioand audio circuits. In a practical design the control may operate insmall degrees alternately on the radio and audio circuits, but in amanner such that it would-be substantially simultaneous. For example,the switching mechanism in Fig. 1 might consist of six contacts insteadof four as shown, and arranged so as to alternately change thetransmission frequcncy band of radio and audio circuits for adjacentswitch positions.

The tap 50 is connected by lead 50' to the low potential side of thegrid circuit of the second audio amplifier. The shaft section betweentaps 4 and 56 is made of insulation material. The contacts and tap ofthe tone control device can be constructed as in the case of thepreceding contacts and taps. The shaft may termimate in a knob whichprotrudes from the receiver operating panel; this knob can be adjustedto receive strong or weak stations; local or distant stations; or to cutout interference from strong adjacent channels. In other words, theselectivity control is used wherever it is desired to change from goodfidelity when receiving strong signals with little interference to highselectivity when receiving weak signals through much interference.

. ploy'ed in place of that shown in Fig. -1.

TIn'Fig. 4 is shown another variable selectivity control deviceconstruction which maybe emused for the A'-B' positions of tap 4; theother strip is used for the C'-D positions of the tap.

'Tap 5%). slides over a resistor Ill, and movement along the resistorvaries the flow of the higher audio frequency currents through the pathincluding resistor 79 and condenser II.

r The tap 59 is connected by lead 12 to the grid of the audio amplifier;the condenser H is grounded. A common mechanical element 30 is arrangedto simultaneously vary taps 4, 4 and 5! in the same manner'as in Fig. 1.The extreme A1 to Di'positicns on resistor H! are shown; themovement oftap 5%] beyond point D1 on resistor Ill results in a further lay-passingof the audio frequencies. ,The circuit connections to the pairs ofstrips are believed obvious from Fig. 1, and are, therefore, not shown.The mechanical assembly of resistor Hi, the pairs of strips and taps.50, 4, 4' can be compact, and may assume different forms.

It will be understood that the invention is not limited to asuperheterodyne type of receiver; it'may be' applied to the radiofrequency and audio frequency networks of a T. R. F. type of receiver.Again, a multi-function tube of the V 55 type, or 2137 type, may be usedto perform the functions of the diode 26 and the first audio amplifier.

While I have indicated and described several systems for'carrying myinvention into effect, it will be apparent to one skilled in the artthat my inventionis by no means limited to the particular organizationsshown and described, but

that many modifications may be made without departing from the scope ofmy invention, as set forth in the appended claims.

'What I claim is: i

1. In combination with a radio receiver of th type including a radiofrequency signal transmission network and an audio frequency signaltransmission network, means for adjusting the degree of selectivity ofsaid radio frequency network, means for adjusting the transmission ofthe higher audio frequencies through the audio network, means'forvarying both said adjusting means, said adjusting means beingconstructed and arranged in such a manner that said varying means isadapted to vary both adjusting means for a portion of the varying meansadjusting range and solely said audio adjusting means for anotherportion of said range.

2. In a system of the type defined in claim 1, means for tuning saidradio frequency network through a desired signal frequency range, andsaid varying means being adjustable independent of said tuning means.

3. In a receiver of the superheterodyne type which includesanintermediate frequency transmission network, a second detector and anaudio frequency transmission network, means for adjusting the resonancecurve characteristic of said intermediate frequency network, means forvarying the audio frequency transmission efii' ciency through said audionetwork, means for simultaneously varying both said adjusting means,said intermediate frequency network ineluding at least two cascadedband. pass circuits,

said resonance curve adjusting means including a selectivity controlleddevice operatively associated with each band pass circuit andconstructed and arranged to vary the sharpness of tuning of said band.pass circuits in overlapping relation.

4. In a receiver of the superheterodyne type which includesanintermediate frequency transmission network, a second detector and anaudio frequency transmission network, means for adjusting the resonancecurve characteristic of said intermediate frequency'network, means foradjusting the audio frequency transmisison efficiency through said audonetwork, and a common means for varying said adjusting means, said bothadjusting means being constructed and arranged to permit the commonmeans concurrently to vary them for a portion of the varying meansadjusting range andsolely said audio adjusting means for another portionof said range said audio transmission adjusting means being "constructedto vary the transmission of the higher audio frequencies through saidaudio network, and said resonance curve adjusting means beingconstructed to change the width of the resonance curve characteristic ina predetermined sense with a change in' transmission of the higher audiofrequencies.

5. In a superheterodyne receiver of the type including a pair of coupledresonant circuits, each circuit being tuned 'to the operatingintermediate frequency, a succeeding pair of coupled resonant circuitssimilarly tuned to the operating intermediate frequency, a seconddetector of the. diode type having its anode connectedto an intermediatepoint on the coil of the second tuned circuit of the succeeding pair ofcoupled circuits, an audio amplifier following the second detector,means including an adjustable element for varying the by-pa'ssing ofaudio frequencies from said audio amplifier, a variable selectivitycontrol device operatively associated with one of the tuned circuits ofsaid first pair of coupled circuits and including an adjustable elementfor changing the degree of selectivity of said cou, pled circuits, asimilar selectivity control device operatively associated with saidsucceeding pair of coupled circuits and including an adjustable elementarranged to change the degree of selectivity of the second pair ofcircuits in an over lapping relation with respect to the selectivityadjustment of the first pair of coupled circuits, and a commonmechanical adjusting means operatively associated with the adjustableelements of said selectivity control devices and said audio amplifieradjustable element, said audio frequency bypassing means including aresistor and condenser in series'across the input circuit of. said audioamplifier, and said adjustable element of the bypassing means beingslidably engaged with the resistor.

6. In a superheterodyne receiver of the type including a pair of coupledresonant-circuits, each circuit being tuned to the operatingintermediate frequency, a succeeding pair of coupled resonant circuitssimilarly tuned to the operating intermediate frequency, means forsuccessively varying the co-efficient of coupling in each pair ofcoupled circuits for regulating the selectivity of the circuits, asecond detector of the diode type having its anode connected to anintermediate point on the coil of the second tuned circuit of thesucceeding pair of coupled'circuits, said .point being chosen to lessenthe loading effect of the diode on the second tuned circuit.

*7. In a radio receiver of the type provided with at least two cascadedradio frequency networks, a detector and an audio network, theimprovement which comprises each of said radio networks consisting of apair of reactively coupled tuned circuits each resonated to a commoncarrier frequency, the coupled pairs of tuned circuits having likeselective resonance curves, a separate auxiliary reactive couplingdevice operatively associated with each pair of coupled tuned circuitsto provide a less selective resonance curve, a switch connected with theauxiliary coupling device of the first pair of tuned circuits, saidswitch including an adjustable element constructed to have a pluralityof adjustment positions, solely the first of the positions correspondingto coupling connection of said first auxiliary coupling device, a secondswitch connected with the auxiliary coupling device of the second pairof tuned circuits, the second switch including an adjustable elementconstructed to have a plurality of adjustment positions, the first ofthe second switch positions, and at least one position subsequentthereto, corresponding to coupling connection of the second auxiliarycoupling device, a common actuating means for concurrently adjusting theadjustable elements of both switches whereby said resonance curves areof different sharpness upon adjustment of said switch elements from saidfirst positions to the succeeding position.

8. In a radio receiver of the type provided with at least two cascadedradio frequency networks, a detector and an audio network, theimprovement which comprises each of said radio circuits having likeselective resonance curves, a separate auxiliary reactive couplingdevice operatively associated with each pair of coupled tuned circuitsto provide a less selective resonance curve, a switch connected with theauxiliary coupling device of the first pair of tuned circuits, saidswitch including an adjustable element constructed to have a pluralityof adjustment positions, solely the first of the positions correspondingto coupling connection of said first auxiliary coupling device, a secondswitch connected with the auxiliary coupling device of the second pairof tuned circuits, the second switch including an adjustable elementconstructed to have a plurality of adjustment positions, the first ofthe second switch positions, and at least on position subsequentthereto, corresponding to coupling connection of the second auxiliarycoupling device, a common actuating means for concurrently adjusting theadjustable elements of both switches whereby said resonance curves areof different sharpness upon adjustment of said switch elements from saidfirst positions to the succeeding position, an audio tone control deviceoperatively associated with the audio network, said tone controlcomprising a series resistorcondenser path shunted across the network tobypass high audio frequency components, means for adjusting theefiective resistance in said path, said adjusting means havingadjustment positions to correspond to the positions of the adjustableelements of said two switches, said adjusting means being coupled tosaid common actuating means, and said adjusting means being constructedto provide additional bypassing of high audio frequency componentssubsequent to the last positions of said adjustable elements.

WENDELL L. CARLSON.

